1. Field of the Invention
This invention relates to a method and system for transporting and processing sour fluids, such as sour natural gas, that may be collected in remote locations.
2. Description of Related Art
Acid fluids are fluids that contain significant amounts of acidic gases such as hydrogen sulfide or carbon dioxide. One type of acid fluid that is encountered in the oil and gas industry is sour natural gas, i.e., natural gas that contains a significant amount of hydrogen sulfide. Sour natural gas, which can be collected from naturally occurring subsurface hydrocarbon reservoirs, is typically transported through a pipeline to a gas processing plant. At the gas processing plant, the sour natural gas is subjected to a sweetening process for removing at least some of its content of the hydrogen sulfide and/or carbon dioxide.
It is often impractical to provide a gas processing plant at the location where the sour gas is collected. For example, in some cases, the sour natural gas is collected from a reservoir in an area that is not convenient for the location of a gas processing plant. Further, it may be impractical to provide a gas processing plant for a single reservoir or well location. Instead, it may be desirable to use one gas processing plant to receive and process sour natural gas from multiple collection areas or reservoirs that are remote from one another, such that the gas processing plant is located far from one or more of the collection areas. Thus, the sour natural gas may be transported over long distances before being processed in the gas processing plant.
One such conventional system for transporting and processing sour natural gas hydrocarbons is shown in FIG. 1. The illustrated system includes four collection locations, each being located near a reservoir. Each collection location includes a metering station and a gas dehydration unit. Natural gas collected at each location is received and metered in the associated metering station. Water is removed in the gas dehydration unit, and the resulting dry, sour natural gas then flows through a pipeline to the gas processing plant. As illustrated, a single pipeline can receive sour natural gas from the various collection locations and provide a combined flow of the dry, sour natural gas to the gas processing plant.
The gas processing plant typically includes equipment for performing a sweetening operation to remove the acids from the gas. For example, the sweetening process can be performed by an absorber at the plant that uses a chemical and/or physical solvent to remove one or more acidic gases from the sour natural gas. The absorber typically receives the solvent with little sour gas content (i.e., as a sour gas-lean solvent) and mixes the solvent with the sour natural gas so that the solvent absorbs the acids from the sour gas, thereby forming a sweetened natural gas and a solvent with an increased content of sour gas (i.e., a sour gas-rich solvent). The sour gas-rich solvent is then delivered to a regeneration facility where the acid is removed from the solvent, thereby regenerating the solvent. The regenerated solvent can be reused (as sour gas-lean solvent), and the removed acid can be further processed, e.g., by sending removed sulfur to a sulfur forming plant that generates a usable sulfur output product.
In one particular system, the hydrogen sulfide concentration in the sour natural gas can be relatively high, up to about 15% or more, and the carbon dioxide concentration can be high as well, up to about 10% or more. Each collection location can be a considerable distance, perhaps up to 35 km, from the gas processing plant, and the collection locations can be in, and/or separated from the gas processing plant by, mountainous and/or densely populated areas. The sour natural gas can be transported through the pipeline at high pressures to the gas processing plant, where the sour natural gas is sweetened and sulfur is recovered.
The transportation of sour fluids can require special considerations, especially when the sour fluids are transported over long distances. For example, some metals are prone to sulfide stress cracking when exposed to hydrogen sulfide, so a conventional system for transporting sour fluids may require equipment made of particular metal alloys. Also, since hydrogen sulfide can be toxic to humans, an unintended release of the sour fluids to the environment may be undesirable, particularly in certain areas, such as densely populated regions. Accordingly, conventional systems typically include a variety of valves or other mechanical devices to limit the volume of gas that could be released in the event of a failure of any portion of the system. For example, the system of FIG. 1 would typically include isolation block valves at various locations along the pipeline and at each of the collection locations. The valves are normally open when the system is operating normally. If a failure occurs, e.g., a burst or other leak in the pipeline, some or all of the valves can be closed manually or automatically to isolate the portion of the system where the failure occurred and prevent additional release of gas.
While the use of valves can greatly improve the safety of the system, the valves can also complicate the construction and/or operation of the system, increase the costs of operation and/or maintenance of the system, and introduce additional points of possible failure. Thus, a continued need exists for an improved system for transporting and processing sour fluids.